The importance of optical data transmission, especially across long distances, has grown enormously over the past few decades.
Long-range, so-called wave division multiplex systems (WDM) having optical amplifiers form the basis of high-capacity transmission networks, so-called backbone networks, in the long-distance range. In a WDM system, a multitude of carrier signals having different wavelengths is transmitted simultaneously via the same optical fiber. Typical distances that are able to be bridged in an optically transparent manner are in the range of 200 km to 2000 km. Optical transmission systems are utilized to guide the data to the backbone networks; these optical transmission systems are likewise based on glass fiber and are partly based on WDM technology as well, but otherwise transmit only one optical channel per glass fiber in most cases. The connection network and feed are jointly based on a tightly meshed glass fiber infrastructure having typical “radii of the network meshes” of 2 to 7 km. The glass fibers may be installed subterraneanously, i.e., in the ground, or above ground such as across bridges, on the floor of bodies of water, in particular transoceanically or in unsupported fashion.
For example, in the case of a subterranean glass-fiber infrastructure, the glass-fiber cables may be destroyed by external causes. A main cause of such destruction can be construction work. However, mechanical influences that have a potentially destructive effect on cables may occur even in the case of submarine cables.
One may determine faults in a fiber with the aid of so-called available optical time domain reflectrometry (OTDR). In the process, a signal is sent into the fiber and its reflections are measured. However, this method has a low sensitivity. And, faults can be detected only once the fiber has already sustained significant damage. Furthermore, OTDR may be employed to check only short sections of a transmission path, typically up to a maximum of 50 km, due to the fact that work between the amplifiers may be carried out only sectionally because of the reflection measurements.